Anatomy and applied aspects of Pleura and Lungs

Pleura
Each lung is enclosed in a serous pleural sac consisting two continuous membrane called visceral and parietal pleura. The two layers are continuous with each other around the hilum of the lung and enclose a potential space between them known as the pleural cavity. The pleura are lined by mesothelium. The visceral pleura invest all the surfaces of the lung forming their shiny outer surface and the parietal pleura lines the pulmonary cavities. The pleural cavity contains a layer of serous pleural fluid, which lubricates the pleural surfaces and allows the layers of pleura to slide smoothly over each other during respiration. Its surface tension also provides the cohesion that keeps the lung surface in contact with the thoracic wall.

The visceral pleura or pulmonary pleura closely covers the lung and is adherent to all its surfaces, including the surfaces within the horizontal and oblique fissures. The visceral pleura dip into the lung fissures so that the lobes of the lung are also covered with it. The visceral pleura are continuous with the parietal pleura at the hilum of the lung.

The parietal pleura line the pulmonary cavities. The parietal pleura consists of four parts

• 1.Costal pleura– covers the internal surfaces of the thoracic wall.( ribs and intercostals spaces)
• 2.Mediastinal pleura– covers the lateral aspects of the mediastinum.
• 3.Diaphragmatic pleura– covers the superior or thoracic surface of the diaphragm on each side of the mediastinum
• 4.Cervical pleura (pleural cupula or dome of pleura)- extends from the superior thoracic aperture into the root of the neck, forming a cup shaped pleural dome over the apex of the lung.

The pleural cavity is the potential space between the layers of pleura and contains a capillary layer of serous pleural fluid, which lubricates the pleural surfaces and allows the layers of pleura to slide smoothly over each other during respiration.

The pulmonary ligament 

• The parietal pleura extends downwards beyond the root as a fold called the pulmonary ligament.

Recesses of pleura
There are two folds or recesses of parietal pleura which act as reserve spaces   for the lung to expand during deep inspiration.

• Costomediastinal recess- lies anteriorly, behind the sternum and costal cartilages.
• Costo diaphragmatic recess – lies inferiorly between the costal and diaphragmatic pleura.

Blood supply and lymphatic drainage of pleura
The pleura is supplied by intercostals, internal thoracic and musculophrenic arteries. The veins drain into the azygous and internal thoracic veins. The lymphatics drain into the intercostals, internal mammary, posterior mediastinal and diaphragmatic nodes.

Nerve supply of pleura
The parietal pleura is supplied by intercostals and phrenic nerves. The parietal pleura is pain sensitive. The pulmonary pleura is supplied by autonomous nerves derived from the spinal segments T4 and T5. This part of the pleura is not sensitive to pain.

APPLIED ANATOMY

Pleuritis or pleurisy
This is the inflammation of the pleura. Acute pleuritis is marked by sharp, stabbing pain, especially on exertion.

Pneumothorax
This is a condition with the presence of air in the pleural cavity. Entry of air into the pleural cavity, resulting from a penetrating wound of the parietal pleura or rupture of a lung results in partial collapse of the lung.

Hemothorax
Presence of blood in the pleural cavity. Hemothorax results more often from injury to a major intercostal vessel than laceration of lung

Hydrothorax
This is a condition with accumulation of significant amount of fluid in the pleural cavity.

Injuries to the pleura
The cervical pleura reaches relatively higher levels in infants and young children because of the shortness of their necks. Consequently, the pleura is vulnerable to injury during the first few years.

The small areas of pleura exposed in the costo vertebral angles inferiomedial to the 12th ribs are posterior to the superior poles of kidneys. The pleura is in danger here from an incision in the posterior abdominal wall when surgical procedures expose a kidney.

Pleural adhesion
If the parietal and visceral pleura adhere the lymphatic vessels in the lung and visceral pleura drains into the axillary lymph nodes. The presence of carbon particles in these nodes is presumptive evidence of pleural adhesion.

Parencentesis thoracic
Aspiration of any fluid from the pleural cavity is called parencentesis thoracis. It is usually done in the 6^th intercostal space in the midaxillary line. The needle is passed through the lower part of the space to avoid injury to the principal neurovascular bundle.

ANATOMY AND APPLIED ASPECTS OF THE LUNGS

Synonym: pulmones
The lungs are the essential organs of respiration; they are two in number, placed one on either side within the thorax, and separated from each other by the heart and other contents of the mediastinum. The lungs are spongy in texture. In the young the lungs are brown or grey in color. Gradually they become mottled black because of the deposition of the inhaled carbon particles. The right lung weighs about 625 g, it is about 50 g heavier than the left lung.

Each lung is conical in shape. It has

• An apex at the upper end
• A base resting on the diaphragm
• Three borders- anterior, posterior and inferior
• Three surfaces- costal medial and diaphragmatic
• The apex is rounded and rises into the root of the neck about 1 inch above the level of the middle third of the clavicle. The structures associated with it are the first rib, blood vessels and nerves in the root of the neck.
• The base is concave and semi lunar in shape. It rests on the diaphragm which separates the right lung from the right lobe of the liver, and the left lung from the left lobe of the liver, fundus of the stomach, and the spleen.
• The anterior border is very thin. It is shorter than the posterior border. On the right side it is vertical and corresponds to the anterior line of pleural reflection. The anterior border of the left lung shows a wide cardiac notch below the level of 4^th costal cartilage. The heart and the pericardium are uncovered by the lung in the region of this notch.
• The posterior border is where the costal and the mediastinal surfaces meet posteriorly. It is thick and ill defined. It corresponds to the medial margin of the heads of the ribs.
• The inferior border circumscribes the diaphragmatic surface of the lung and separates the base from the costal and the medial surfaces.
• The costal surface is convex and is closely associated with the costal cartilages, the ribs and the intercostals muscles.
• The medial surface is concave and has a roughly triangular shaped area, called the hilum, at the level of 5^th, 6^th and 7^th thoracic vertebrae. This surface is divided into a posterior or vertebral part and an anterior or mediastinal part.
• Diaphragmatic surface forms the base of the lung rests on the dome of the diaphragm. The concavity is deeper in the right lung because of the higher position of the right diaphragmatic dome, which overlies the large liver.

Root of the lung
The lungs attach to the heart and trachea by structures that comprise the roots of the lungs. The root of the lung is formed by structures entering and leaving the lung at its hilum. The root is enclosed within the area of continuity between the parietal and visceral layers of pleura called the pleural sleeve or mesopneumonium or mesentery of the lung. The hilum of the lung is the area on the medial surface of each lung, the point at which the structures forming the root.

 These include:

• bronchus
• pulmonary artery
• pulmonary veins
• bronchial artery
• Bronchial veins
• Lymph vessels
• Parasympathetic and sympathetic nerves.

The area between the lungs is the mediastinum. It is occupied by the heart, great vessels, trachea, right and left bronchi, esophagus, lymph nodes, lymph vessels and nerves.

Fissures and Lobes of the lungs 

The right lung is divided into three distinct lobes by two fissures( oblique and horizontal)

• Superior
• Middle
• Inferior

The left lung is smaller as heart is situated left of the midline. It is divided into only two lobes by the oblique fissure.

• Superior
• Inferior

The oblique fissure cuts into the whole thickness of the lung except at the hilum. It passes obliquely downwards and forwards, crossing the posterior border about 2 ½ inches below the apex and the inferior border about 2 inches from the medial plane. Due to the oblique plane of the fissure the lower lobe is more posterior and the upper lobe more anterior.

In the right lung, the horizontal fissure passes from the anterior border up to the oblique fissure and separated a wedge shaped middle lobe from the upper lobe. The fissure runs horizontally at the level of 4^th costal cartilage and meets the oblique fissure in the mid-axillary line. The tongue shaped projection of the left lung below the cardiac notch is called the lingula. It corresponds to the middle lobe of the right lung.

The lungs expand maximally in the inferior direction because movements of the thoracic wall and the diaphragm are maximal towards the base of the lung. The presence of the oblique fissure of each lung allows a more uniform expansion of the whole lung.

Interior of lung
The lungs are composed of the bronchi and smaller air passages, alveoli, connective tissue, blood vessels, lymph vessels and nerves.

Trachea and bronchi
The main bronchi or the primary bronchi, one to each lung, pass inferomedially from the bifurcation of the trachea at the level of the sternal angle to the hila of the lungs. The walls of the trachea and bronchi are supported by horseshoe or C- shaped rings of hyaline cartilage.

The right main bronchus is wider, shorter and runs more vertically than the left main bronchus as it passes directly to the hilum of the lung.

The left main bronchus passes inferolaterally, inferior to the arch of aorta and anterior to the esophagus and thoracic aorta, to reach the hilum.
The main bronchi enter the hila of the lungs and branch in a constant fashion within the lungs to form the bronchial tree. Each main bronchus divides into lobar bronchi (secondary bronchi), two on the left and three on the right, each of which supplies a lobe. Each lobar bronchus divides into several segmental bronchi (tertiary bronchi) that supply the bronchopulmonary segments.

Bronchopulmonary segments
These are well defined sectors of the lung, each one of which is aerated by a tertiary or segmental bronchus. Each segment is pyramidal in shape with its apex directed towards the root of the lung and its base at the pleural surface. It is the largest subdivision of a lobe. It is separated from adjacent segments by connective tissue septa. It is supplied independently by a segmental bronchus and a tertiary branch of pulmonary artery. It is named according to the segmental bronchus supplying it. It is drained by the intersegmental parts of the pulmonary veins that lie in the connective tissue between and drain adjacent segments. It is surgically respectable.

Beyond the direct branches of the lobar bronchi, there are 20-25 generations of the branches that eventually end in terminal bronchioles. Each terminal bronchiole give rise to several generations of respiratory bronchioles and each respiratory bronchiole provides 2 to 11 alveolar ducts, each of which give rises to 5 or 6 alveolar sacs lined by alveoli.

Pulmonary unit
Each respiratory bronchiole aerates a small part of the lung known as a pulmonary unit. The respiratory bronchiole ends in microscopic passages like- alveolar ducts, atria, air succules and pulmonary alveoli. The alveolus is the basic structural unit of gas exchange in the lung. New alveoli continue to develop until the age of 8 yrs, by which there are about 300 million alveoli.

Arterial supply of the lungs
Each lung has a large pulmonary artery supplying blood to it and two pulmonary veins draining blood from it. The right and left pulmonary arteries arise from the pulmonary trunk at the level of the sternal angle and carry venous blood to the lungs for oxygenation. Each pulmonary artery becomes part of the root of the corresponding lung and gives off its first branch to the superior lobe before entering the hilum. Within the lung each artery descends posterolateral to the main bronchus and divides into lobar and segmental arteries. Consequently, an arterial branch goes to each lobe and bronchopulmonary segments. The artery and the bronchi are paired in the lung, branching simultaneously and running parallel courses.

The left bronchial arteries arise from the thoracic aorta and the right bronchial artery may arise from a superior posterior intercostals artery or from the thoracic aorta or from a left superior bronchial artery.

Venous drainage of the lungs
Usually there are two bronchial veins on each side carrying oxygenated blood from the lungs to the left atrium of the heart. The right bronchial vein drains into the azygos vein. The left bronchial vein drains either into the left superior intercostals vein or into the hemiazygos vein. The greater part of the venous blood from the lung is drained by the pulmonary veins. Beginning in the pulmonary capillaries, the veins unite into larger and larger vessels.

Lymphatic drainage of the lungs 

• There are two sets of lymphatics, both of which drain into the bronchopulmonary nodes.
• Superficial vessels drain the peripheral lung tissue lying beneath the pulmonary pleura. The vessels pass round the borders of the lung and margins of the fissures to reach the hilum.
• Deep lymphatic drain the bronchial tree, the pulmonary vessels and the connective tissue septa. They run towards the hilum where they drain into the bronchopulmonary nodes.

Nerve supply of the lungs 

Parasympathetic nerves are derived from the vagus. These fibres are

• Motor to the bronchial muscles, and on stimulation cause bronchospasm
•  Secretomotor to the mucus glands of the bronchial tree
• Sensory fibres are responsible for the stretch reflex of the lungs and for the cough reflex.
• Sympathetic nerves are derived from spinal segments T2 to T5. These are inhibitory to the smooth muscle and the glands of the bronchial tree.
• Both parasympathetic and the sympathetic nerves first form anterior and posterior pulmonary plexuses situated in front of and behind the lung roots. From the plexus, nerves are distributed to the lungs along the blood vessels and bronchi.

Surface anatomy of the pleurae and lungs

• The cervical pleurae and the apices of the lungs pass through the superior thoracic aperture into the supraclavicular fossa, which are located superior and posterior to the clavicles and lateral to the tendons of the sternomastoid muscles.
• The anterior border of the lungs lies adjacent to the anterior line of reflection of the parietal pleura as far inferiorly as the 4^th costal cartilages. The anterior border of the left lung is more deep indented by its cardiac notch. On the right side, the pleural reflection continues inferiorly from the 4^th to the 6^th costal cartilage, paralleled closely by the anterior border of the right lung.
• The pleural reflections reach the MCL at the level of 8^th costal cartilage, the 10^th rib at the MAL, and 12^th rib at the scapular line.
• The inferior margins of the lungs reach the MCL at the level of 6^th rib, MAL at 8^th rib and the scapular line at the 10^th rib.
• The oblique fissure of the lungs extends from the level of the spinous process of the T2 vertebra posteriorly to the 6^th costal cartilage anteriorly.
• The horizontal fissure of the right lung extends from the oblique fissure along the 4^th rib and costal cartilage anteriorly. 

APPLIED ANATOMY

Variation in the lobes of the lung
Occasionally an extra fissure divides a lung or a fissure is absent. The most common accessory lobe is the azygos lobe, which appears in the right lung in about 1% of people. In these cases, the azygos vein arches over the apex of the right lung and not over the right hilum, isolating the medial part of the apex as an azygos lobe.

Aspiration of foreign bodies
Since the right bronchus is wider and shorter and runs more vertically than the left bronchus, foreign bodies are more likely to enter and lodge in it or in one of its branches.

Bronchoscopy
When examining with a bronchoscope, one observes a keel like ridge called the carina. It is a cartilaginous projection of the last tracheal ring. Morphological changes in the carina are important diagnostic signs to bronchoscopists in assisting with the differential diagnosis of respiratory disease.

Lung resection
Knowledge of the anatomy of the bronchopulmonary segments is essential for precise interpretation of radiographs and other diagnostic images of the lungs and for surgical resection of the diseased segments. Bronchial and pulmonary disorders such as tumors or abscesses often localize in a bronchopulmonary segment, which may be surgically resected. During treatment of the lung cancer, the surgeon may remove a whole lung (pneumonectomy), a lobe (lobectomy) or a segment (segmentectomy).

Bronchial asthma
Bronchial asthma IS n increasingly common condition of the lungs in which widespread narrowing of the airways is present. Asthma is caused by varying degrees by contraction of the smooth muscle, edema of the mucosa, and mucus in the lumen of the bronchi and bronchioles. These changes are caused by the local release of spasmogens and vasoactive substances influencing the tone and caliber of the blood vessels (eg; histamine and prostaglandins). The absence of cartilages in the walls of bronchioles is a potential hazard. In asthma, there is difficult expiration because the bronchioles which are opened during inspiration also have to remain open during expiration if they are to permit a rapid outflow of air owing to elastic recoil of the lung tissues. That’s why more wheeze and breathing difficulties are experienced during expiration.

Pulmonary thromboembolism
Obstruction of a pulmonary artery by a thrombus is a common cause of morbidity and mortality. An embolus in a pulmonary artery forms when a thrombus, fat globule, or air bubble travels in the blood to the lungs from a leg vein. The thrombus may block a pulmonary artery or its branch. The immediate result is partial or complete obstruction of the blood to the lungs. When a large embolus occludes a pulmonary artery, the patient suffers from acute respiratory distress and may die in a few minutes.

Floatation of the lungs
Fresh healthy lungs always contain some air. Diseased lungs filled with fluid, fetal lungs, and lungs from a still born infant will not float. The lungs of a live born infant who dies shortly after birth floats. These observations are of medico legal significance in determining whether a dead infant was stillborn or whether it was born alive and started to breathe.

Pneumonia

• This occurs when protective processes fail to prevent inhaled or blood born microbes reaching and colonizing the lungs.
• Types- lobar pneumonia and bronchopneumonia

Lobar pneumonia
This is the infection of one or more lobes by str.pneumoniae mainly type 1, 2 and 3. The infection leads to the production of watery inflammatory exudate in the alveoli. This accumulates and fills the lobule then overflows into the adjacent lobules spreading the microbes. It is of sudden onset and pleuritic pain accompanies inflammation of the visceral pleura. If not treated, the disease goes through a series of stages followed by resolution and reinflation of the lobes in 2-3 weeks.

Bronchopneumonia
Infection is spread from the bronchi to terminal bronchioles and alveoli. As these become inflamed, fibrous exudates accumulates and there is an influx of leukocytes. Small foci of consolidation develop. There is frequently incomplete resolution with fibrosis. Bronchiectasis is a common complication leading to further acute attacks. Bronchopneumonia occurs more commonly in infancy and old age, and death is fairly common.

Microbes causing pneumonia

1. Staphylococcus aureus
Infection is preceded by influenza, measles, whooping cough or chronic lung disease. Incomplete resolution may cause abscess formation.

2. Friedlander’s bacillus (klebsiella pneumoniae)
This commensal is sometimes present in the upper respiratory tract, especially where there is advanced dental caries. It commonly causes pneumonia in men over 50 years and is often associated with diabetes mellitus and alcoholism.

3. Legionella pneumonophila
These microbes are widely seen in water tanks, shower heads and air conditioning systems. They may cause severe form of pneumonia   (Legionnaires ’ disease), complicated by GI disturbances, headache, mental confusion and renal failure.

4. Streptococcus pyogenes
Infection usually preceded by influenza or measles. In severe cases death may occur within few days.

5. Pseudomonas pyocyanae
 This is a commensal in the bowel that may cause a type of pneumonia acquired by cross infection in hospitals especially in patients with mechanically assisted ventilation or tracheostomy.

6. Streptococcus pneumonia (pneumococcus)
This is a commensal in the respiratory tract which may cause lobar pneumonia or bronchopneumonia, usually preceded by viral infection.

Lung abscess
Local suppuration and necrosis within the lung substance is most commonly caused by str.viridans, str. Pyogenes, str. Pneumoniae, staph. Aureus.

Mycobacterium tuberculosis
Man is the main host. The microbes cause pulmonary tuberculosis and are spread either by droplet infection from an individual with active tuberculosis, or in dust contaminated by infected sputum.

Primary tuberculosis
When microbes are inhaled they colonize a lung bronchiole, usually towards the apex of the lung. There may be no evidence of clinical disease during the initial stage of non- specific inflammation. T- Lymphocytes respond to the microbes and the individual becomes sensitized. Macrophages surround the microbes at the site of infection forming Ghon foci or tubercles. Some macrophages containing live microbes are spread in lymph and infect hilar lymph nodes. So primary complex consist of the Ghon foci and infected hilar lymph nodes. Primary tubercle is usually asymptomatic. There are various outcomes:

• The disease may be permanently arrested, the foci becoming fibrosed and calcified.
• Microbes may survive in the foci and become the source of post primary infection months or years later.
• The disease may spread throughout the lung or to other parts of the body. The disease spreads to other parts of the body via lymph and blood leading to wide spread of the infection and the development of numerous small foci throughout the body ( miliary tuberculosis).

Secondary or Post primary tuberculosis : This phase occurs only in people previously sensitized by a primary lesion.

Acute Bronchitis
This is usually a secondary bacterial infection of the bronchi. It is usually preceded by a common cold or influenza.

Chronic bronchitis
It is defined clinically as the presence of cough with sputum for 3 months in 2 successive years. It is a progressive inflammatory disease resulting from prolonged irritation of the bronchial epithelium. It develops mostly in middle aged men who are chronic heavy smokers and may have a familial predisposition. Ventilation of the lung is severely impaired, causing breathlessness, leading to hypoxia, pulmonary hypertension and right heart failure.

Bronchiectasis
There is permanent abnormal dilatation of the bronchi and bronchioles. It is associated with chronic bacterial infection and there may be history of childhood bronchiolitis and bronchopneumonia, cystic fibrosis or bronchial tumour. The lower lobe of the lung is usually affected. Suppuration is common.

Bronchial carcinoma
Primary bronchial carcinoma is a common form of malignancy. The tumour usually develops in a main bronchus, forming a large friable mass that project into the lumen sometimes causing obstruction. As the tumour grows it may erode a blood vessel causing hemoptysis. The cause is not known but there is strong positive association with cigarette smoking and passive smoking.

The modes of spread are infiltration of local tissues and the transport of tumour fragments in blood and lymph. Local spread may be within the lung, to the other lung or to the mediastinal structures. Tumour fragments may spread along the lymph vessels to the successive lymph nodes in which they may cause metastatic tumours. The most common sites of the blood born metastases are liver, brain, adrenal glands, bones and kidneys. 

Atelectasis
There is defect in the expansion of the lungs. There are two types based on the time of onset

• Immediately after birth
•  Within minutes or hours of birth

Pneumoconiosis
This is a group of lung diseases caused by inhaling organic or inorganic atmospheric pollutants. Some of them are-

• Coal workers pneumoconiosis- soft bituminous coal
• Siilicosis- dust containing silicon compounds
• Asbestosis – asbestos
• Pleural mesothelioma- asbestos
• Byssinosis – cotton fibres

Extrinsic allergic alveolitis

This is a group of conditions caused by inhaling materials contaminated by moulds and fungi.

• Farmers lung- mouldy hay
• Bagassosis- mouldy sugar waste
• Bird handler’s lung- mould in bird droppings
• Malt worker’s lung- mouldy barley

Reference:

• Clinically Oriented Anatomy – Kieth L Moore
• Anatomy and Physiology in health and illness- Ross and Wilson
• B.D.Chaurasia’s Human Anatomy, regional and applied.
• Anatomy of human body- Henry Grey